scons: fix library installation directory

On gentoo systems SCons setup.py tries to install into
$(HOST_DIR)/usr/lib64 but SCons looks up for its stuff in
$(HOST_DIR)/usr/lib instead hence breaking.
Make it install into $(HOST_DIR)/usr/lib by force to avoid the issue, it
shouldn't matter on other distributions.
Version bumping doesn't fix it.

[Peter: wrapped long line]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>